The invention relates to a method for converting aloeresin A to aloesin.
Aloe is a succulent plant of which more than 300 species are known to exist, most of which are indigenous to Africa. Products from the aloe have been used in traditional medicine for centuries and in particular have been used in dermatological applications for the treatment of burns, sores and other wounds. Modern therapeutic observations include anti-inflammatory activity, anti-tumor activity, anti-acid activity, anti-diabetic activity, tyrosinase inhibiting activity and antioxidant activity. Aloe products are also used extensively in the cosmetic and health food industries, especially with the recent increase in popularity of natural products.
The main species of African aloe from which pharmaceutical, therapeutic, dermatological or cosmetic applications are obtained is Aloe ferox, a species restricted to Southern Africa. Other species of aloe are also used for similar purposes. Aloe vera is used extensively in the USA in skin care products, shampoos and health drinks. A. ferox has, however, been found to have several superior properties to A. vera. For example, A. ferox has a higher calcium and total amino acid content than A. vera. The cut leaf of cultivated A. ferox plants also produces approximately 20 times more bitter sap, weight for weight, than an A. vera plant growing beside it. Since A. ferox leaves are much thicker and wider, the total yield of bitter sap per leaf is even greater. The amount of gel recovered from A. ferox is also consistently greater in volume than that obtained from A. vera. 
Many methods exist for the isolation of particular components of the aloe (see, for example, U.S. Pat. Nos. 4,735,935 and 4,656,029). One of the commercial products of the aloe is obtained from the sap produced by the leaf of the aloe and is called aloe bitters due to its bitter taste. The sap is collected and dried by traditional methods which will be known to persons skilled in the art to produce a dark brown solid substance known as aloe bitters or Cape Aloes.
Commercial aloe bitters contains four major constituents, i.e. aloin A, aloin B, aloesin and aloeresin A.
The overall composition of major compounds in A. ferox leaf exudate is remarkably invariable, especially when the morphological variation and wide natural distribution area of this species are considered. In A. ferox, aloeresin A, aloesin and aloin (both epimers A and B) contribute between 70% and 97% of total dry weight, in a ratio of approximately 4:3:2 respectively.
Aloin A and B (or barbaloin) are isolated from aloe bitters, normally by selective extraction, and are principally used as a purgative. Aloesin is also isolated from aloe bitters and is used as a skin lightening and sunscreen agent (U.S. Pat. No. 4,656,029). The aloeresin A is usually discarded. Thus, although aloeresin A is present in the largest quantity in the aloe sap, it has little or no commercial value. It would therefore be advantageous to convert the aloeresin A into a compound which has a higher commercial value.
In the description which follows, any reference to aloe sap is also intended to refer to aloe bitters or an extract of aloe bitters.